There are few depictions of regular sepoys from the middle part of the century. The one line drawing purporting to be of sepoys around the time of Plassey I can think of dates to the mid 19th C and seems to depict the uniform worn from the 1770s/ 1780s onwards with sun dial hats. .
With that in mind and the manner that the EIC frequently raised irregular native cavalry regiments that supplied their own kit and officers, I'd suggest that any mercenaries or 'militia' would likley be wearing local dress and unlikely to be uniformed per se.